The most common method of pumping oil from an oil well is by the use of a downhole liquid pump which is actuated by a rod which is reciprocated from the well surface by a prime mover such as a motor or engine. Generally, the pumping system capacity is in excess of the productivity rate of the oil reservoir. This results in the well being pumped dry or "pumped off" causing fluid pound and damage to the rod string, pump, and possibly the surface equipment. Numerous control systems have been proposed, such as disclosed in U.S. Pat. Nos. 3,953,777, 4,286,925 and 4,487,061, to measure when the well has been pumped off and thereafter shut down the pumping unit for a predetermined amount of time.
However, there are circumstances when it is not desirable to stop the pumping. For example, if the well production includes sand, the sand would settle out when the pumping unit was stopped and clog or damage the unit. Also, if the well is producing a significant amount of water in a very cold climate, the water could turn to ice and damage a stopped pumping unit. Therefore, for these and other reasons, it may not be desirable to stop the pumping unit, but it is also not desirable to pump the well dry and subject the pumping unit to fluid pound and damage.
The present invention is directed to a method and operation for controlling the pumping speed of a rod pumped liquid producing well in which the pump may continue to pump but the pumping speed is varied for preventing the well from being pumped dry. Preferably, the method and apparatus of the present invention is directed to controlling a well pumping unit for maintaining a substantially constant amount of filling of a rod actuated liquid well pump thereby avoiding the problem of pump off or pumping the well dry thereby avoiding also the problem of shutting down the well due to pump off.